Pyelo-ureteritis cystica is a benign and usually asymptomatic disease of the upper urinary tract. Pathologically it consists of small subepithelial cysts which elevate the mucous membrane of the renal pelvis and ureter and appear as round radiolucent areas on urographic studies. Very little has been written about pyelo-ureteritis cystica in the radiologic and urologic literature, where it should receive the greatest attention (1). Since it is quite possible that many physicians have never seen this condition, it may present a differential problem when it does occur. It is our purpose in this paper to discuss the pathology and radiographic characteristics of ureteritis and pyelitis cystica and to review our experience with this disease. Cystitis cystica will also be given some consideration, since it is similar histologically. This condition, however, is almost always diagnosed by cystoscopy and is rarely recognized on the urogram. Histology The upper urinary tract, including the renal pelves, ureters, and bladder, is of similar histologic structure, being composed essentially of three layers. The inner layer of mucous membrane is connected directly to the middle smooth muscle layer by lamina propria. There is no distinct submucosa in man. The outer layer is a coat of connective tissue, the adventitia. The mucosal lining is a transitional epithelium which increases in cell thickness as it approaches the bladder (2). The pathogenesis of pyelo-ureteritis cystica and cystitis cystica is the same, both conditions arising from cystic degeneration of epithelial cell nests (3). The cysts are situated immediately beneath the mucosal lining and elevate it into the lumen (Figs. 1 and 3). In the renal pelvis and ureter the cysts range from microscopic dimensions to as much as 2 cm. in diameter; in the bladder they measure up to 3 cm. (4). In 1,590 routine autopsies reported by Pessin (4), the incidence of ureteritis cystica was 0.05 per cent. Cystitis cystica is a much more common finding. History and Pathogenesis Pyelo-ureteritis cystica was recognized long ago, being first described in 1761 by Morgagni (5), who reported 2 cases. Rayer (6) in 1837, Virchow (7) in 1863, and Rokitansky (8) in 1861 published additional case reports. Litten (9) in 1876, von Limbeck (10) in 1887, and von Brunn (3) in 1893 made the first detailed microscopic examinations in autopsied cases. Over the years several theories have been offered to explain the etiology and pathogenesis of this disease. In 1889 Eve (11) reported a case which he attributed to a parasite. Urquhart (12), more recently, found Schistosoma haematobium in 6 cases of cystitis cystica seen at autopsy. Litten (9) and Virchow (7) suggested inflammatory blockage of glands and crypts in the mucous membrane as the source of the cysts.